The preferred embodiments relate to electric motor systems and methods and, more particularly, to adaptive torque disturbance cancellation in such systems and methods.
Electric motors are implemented in numerous systems and, in many applications, drive a load with a strong periodic torque disturbance. For example, loads that create such a disturbance include a reciprocating compressor, a washing machine, a piston load, and so forth. The changing load torque affects motor speed, which in turn affects current demand in responding to the altered motor speed. Specifically, a load torque increase tends to slow down the motor speed, and, in the prior art, the reduced speed is detected (e.g., by a speed controller) and, in response, current to the motor is increased to counteract the change in load torque. The effectiveness of the speed controller to regulate speed under these conditions, however, may fall short of the value required to achieve the desired speed regulation in most cases, as further detailed below.
FIG. 1 illustration a simplified, abstracted view of a conventional motor control system 10, by way of further introduction. System 10 includes a motor 12 that rotates a shaft 12SH to turn a load 12L, where load 12L is shown merely as a plate on shaft 12SH with the understanding that a larger device may be connected thereto, to rotate either in 1:1 relationship with shaft 12SH or as some multiple, such as through some gearing or other translational system. Motor 12 is connected in a feedback configuration that in general includes a velocity loop and within the velocity loop is a torque loop, both shown in dashed rectangles. A commanded velocity VC signal represents a desired velocity of motor 12 and is input to an error generator 14, which also receives a feedback input as an estimated velocity VE from a velocity estimator 16. Velocity estimator 16 receives one or more signals SM from a sensor(s) 12SR, at motor 12; these signals may represent a motor shaft mechanical (i.e., angular) position θm, or the signals may be current and voltage, from which velocity estimator 16 can estimate the angular velocity VE. The output of error generator 14 is connected to an input of a speed controller 18, which typically applies a function, such as a proportional-integral (PI) control function, to the error signal between commanded velocity VC and estimated velocity VE, with that error signal provided by error generator 14. The output of speed controller 18 is a commanded current IC, which is connected as an input to an error generator 20, which also receives a feedback input as a measured current IM from sensors 12SR at motor 12. The output of error generator 20 is connected to an input of a torque controller 22, which typically applies a function, also such as a proportional-integral control function, to the error signal between commanded current IC and measured current IM, with that error signal provided by error generator 20. In response, torque controller 22 outputs a drive signal SD (or plural signals) which may comprise data signals that ultimately become the drive signal to motor 12.
The general operation of system 10 is readily understood in the art. Commanded velocity VC is input with a goal toward operating motor 12 at that velocity, and the feedback and PI operations thus endeavor toward satisfying that goal. As further background to the preferred embodiments, however, introduced earlier is the notion that a motor load with a strong periodic torque disturbance affects the speed of motor 12. Thus, in FIG. 1, velocity estimator 16 provides estimated velocity VE as feedback that will indicate the reduced speed from the torque disturbance, thereby increasing the error indicated by error generator 14 and causing speed controller 18 to increase the commanded current IC. The effectiveness of speed controller 18 to regulate speed, however, is dependent on its gain, where higher gains result in more constant motor speed. But the speed controller gain is usually limited by the stability requirements of the system, so that limitation may cause speed regulations that still vary undesirably or fail to reach desired levels in certain applications. Further, when the load reaches a different point in the mechanical cycle, the torque decreases, which causes speed controller 18 to increase motor speed. But once again, speed controller 18 cannot respond as quickly as desired, resulting in speed overshoot. This process repeats over and over again, resulting in a cyclical speed disturbance. The derivative of speed is acceleration, and the derivative of acceleration is jerk, which is directly proportional to the mechanical shock to the system. As jerk increases, so does the audible noise and mechanical wear of the system.
Given the preceding discussion, while the prior art approaches may be acceptable in certain implementations, some applications may have requirements that are not sufficiently addressed by the prior art. Thus, the present inventors seek to improve upon the prior art, as further detailed below.